Radical chemistry at night: Comparisons between observed and modelled HOx, NO3 and N2O5 during the RONOCO project

The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) aircraft campaign during July 2010 and January 2011 made observations of OH, HO2, NO3, N2O5 and a number of supporting measurements at night over the UK, and reflects the first simultaneous airborne measurements of these species. We compare the observed concentrations of these short-lived species with those calculated by a box model constrained by the concentrations of the longer lived species using a detailed chemical scheme. OH concentrations were below the limit of detection, consistent with model predictions. The model systematically underpredicts HO2 by similar to 200% and overpredicts NO3 and N2O5 by around 80 and 50 %, respectively. Cycling between NO3 and N2O5 is fast and thus we define the NO3x (NO3x = NO3 + N2O5) family. Production of NO3x is overwhelmingly dominated by the reaction of NO2 with O-3, whereas its loss is dominated by aerosol uptake of N2O5, with NO3 + VOCs (volatile organic compounds) and NO3 + RO2 playing smaller roles. The production of HOx and ROx radicals is mainly due to the reaction of NO3 with VOCs. The loss of these radicals occurs through a combination of HO2 + RO2 reactions, heterogeneous processes and production of HNO3 from OH+NO2, with radical propagation primarily achieved through reactions of NO3 with peroxy radicals. Thus NO3 at night plays a similar role to both OH and NO during the day in that it both initiates ROx radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N2O5 aerosol uptake coefficient (gamma(N2O5)) is discussed and we find that a value of gamma(N2O5) = 0.05 improves model simulations for NO3 and N2O5, but that these improvements are at the expense of model success for HO2. Improvements to model simulations for HO2, NO3 and N2O5 can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.